This is just my experience, but I actually very successfully applied this year with ZERO letters explicitly from professors who taught me. I had 4 science research supervisors, 1 ethics/philosophy research supervisor (all 5 professors at my undergraduate/graduate institution or similar institutions with some teaching responsibilities across), 1 non-science professor with whom I took a course and who served on my graduate committee (kind of a research mentor!), 2 professors for whom I TAed classes (public health)....I had taken their courses previously, 2 clinical/extracurricular supervisors.
Not a single professor explicitly mentioned what I was like as a student in their courses (Yes, I have hence seen all the letters in at least one iteration because they all sent them to me after submitting them), mostly because the relationships were beyond that dynamic. I think there is a lot of flexibility in what you can actually submit as LORs.....especially if all of your LORs are going to knock the committee's socks off.
The hard and fast rules that I would go by are:
1. ALL research mentors
2. Anyone with whom you are going to list a significant experience in which you participated for > 2 years.
3. At least 1 person who routinely teaches SOME undergraduate course work on your campus (I actually did NOT have this....they all taught almost explicitly grad/professional courses)....preferable broken into two science and one non-science.
Additionally, my opinion in that committee letters tend to hurt you (advice from an MD/PhD assoc/assist program director at a top-20 program for whom I worked) because it limits how much any one person can say about you. So, if you have a lot of strong individual letters this will actually be better as long as you can swing it with respect to legality. I for example could have submitted a committee letter because of of my institutions had a committee, but elected not to because I was farther out of undergrad and I did not want to.
However, differnt programs have different preferences as to their students (I have opinions here) so different programs may want to see different things. The only way to know this is to ask, look at students in programs and what their apps looked like, and compulsively stalk to gleen info from SDN etc (which I would not recommend of course!)
Don't stress tooooo much.